1. Field of the Invention
The present invention relates to polyethylene films. The present invention particularly relates to polyethylene films useful in barrier applications.
2. Background of the Art
It is known to use polymer films in packaging applications to act as barriers for stopping or mitigating permeation of water vapor, oxygen, odors, and the like. Such polymers are, for the purposes of the present invention, barrier films, and such applications are, for the purposes of the present invention, barrier applications. For example, it is known that a recyclable multilayer ethylene polymer sheet structure for constructing a package to hold bulk products such as dry pet food having an odoriferous component, the sheet having, from the inner surface contacting the product, to the outer non-product contacting surface the structure: LLDPE/HDPE/LLDPE/BR/LDPE where LLDPE is a linear low density polyethylene, HDPE is a high density polyethylene and BR is an odor barrier resin such as ethylene vinyl alcohol copolymer having odor barrier properties and LDPE is low density polyethylene.
Another example of a barrier application is known in the art. Therein, readily castable high density polyethylene (HDPE) films, preferably containing broad molecular weight distribution HDPE resin having treated upper skin layers and untreated lower skin layers, preferably coextruded, on each side are disclosed. The treated upper skin layer is derived from a polymer, e.g., ethylene-propylene-butene-1 terpolymer, and the lower skin layer is derived from a polymer such as ethylene-propylene-butene-1 terpolymer and LDPE compounded with silicone oil. Such skin layers provide a film which is readily castable while maintaining the desirable properties of HDPE including dead fold, twist retention, TD tear and moisture barrier.
Not all barrier applications use multiple polymers. All polyethylene multilayer films are also known. For example, J. Krohn, W. Todd & J. Culter, OPTIMIZING BARRIER PERFORMANCE OF MULTILAYER POLYETHYLENE FILMS, (2001) published on the Internet at: http://www.equistarchem.com/TechLit/Brochures/Barrier%20Performance%20Multi-Layer%20Film%209366.pdf; discloses an all polyethylene multilayer film having barrier properties.
Also known in the art is improving certain properties of chromium produced polyethylene blow molding resins through the addition of organic peroxides. The improvements were directed towards increasing the stress crack resistance of blow molded polyethylene bottles. It should be noted, however, that the improved results were seen in polyethylene resins having a broad molecular weight distribution, i.e. Mw/Mn being greater than 10 and that these improvements relate to properties associated with blow molding rather than barrier films.
Polyethylene resins produced with Ziegler-Natta catalysts have also been modified through the use of free radical initiators, such as oxygen, to improve the polymer properties. It is known, for example, to effect the modification of polyethylene resins used in films through such means. Ziegler-Natta produced polyethylene resins used in blow molding resins are typically bimodal resins wherein a low molecular weight polymer and a high molecular weight polymer are combined to provide a broad molecular weight distribution to improve the melt properties of the resin.